In recent years, along with increases in numbers of high-rise buildings, cases of radio waves of TV frequency bands such as VHF and UHF being reflected by buildings have become more common. Consequently, ghosting, which arises on a TV screen when radio waves arriving at the antenna of a TV receiver directly from a TV station (direct waves) and radio waves reflected by buildings (reflected waves) are incident on the antenna simultaneously, have become a serious social problem. For this reason, with the object of reducing the number of these radio waves reflected by exterior walls of buildings, radio wave absorbing panels made of magnetic materials such as ferrite have been affixed to or embedded in exterior walls to absorb radio waves incident on buildings.
For window glass installed in building window openings, to reduce air-conditioning cooling loads in summer (to save energy), glass coated with a film having a heat ray reflecting function has been used; however, because films having a high heat ray reflecting capability have low electrical resistance, their reflectivity of radio waves is high and they are a cause of radio wave obstruction.
Radio wave absorbing panels for reducing radio wave reflection in which ferrite is used cannot be applied to window openings of building because ferrite does not transmit visible light. Consequently the situation has been that heat ray reflecting ability has been sacrificed and transparent heat ray reflecting films having relatively high electrical resistance have been coated on windows of buildings to reduce radio wave reflection and prevent radio wave obstruction by transmitting radio waves through window openings into buildings.
Concerning window glass for buildings and vehicles, technology is known (from, for example, Japanese Patent Laid-Open Publication Nos. HEI 3-250797, HEI 5-042623, HEI 5-050548 and HEI 7-242441), whereby, with the object of preventing obstruction due to radio wave reflection of high-performance heat ray reflecting films, high heat ray reflectivity and low radio wave reflectivity are realized at the same time by a conducting film being divided up into areas of a size amply smaller than the wavelength of incident radio waves to raise its radio wave transmittivity.
Radio wave absorbing panels in the related art which have radio wave transmittivity are an attempt to prevent radio wave reflection problems by providing window glass of buildings with radio wave transmittivity; however, associated with these there are the problems that incoming radio waves penetrate to the inside of the building and affect office equipment such as personal computers and that electromagnetic waves radiated from electronic appliances inside the building leak through the window glass to outside the building. Although this kind of radio wave obstruction is expected to increase in the future, no effective countermeasure has been taken besides reflecting and thereby blocking radio waves by using a conducting wire mesh or a conducting film on windows of buildings, and in districts where there is a likelihood of a radio wave reflection obstruction arising it has been difficult to build buildings which have large-area windows and block radio waves.
To solve this problem, it is necessary to create a practically usable transparent panel which absorbs radio waves instead of reflecting or transmitting them.
At present there are radio wave absorbing panels made by disposing in parallel two transparent substrates each coated with a conducting film having a controlled sheet resistivity, with which panels it is possible to realize a high radio wave absorbing capacity by utilizing resonance caused by interference of multiple reflections of radio waves. However, to absorb VHF band (about 100 MHz) radio waves, the gap between the two substrates constituting the radio wave absorbing panel must be made from several tens of cm to over a meter, and therefore such panels cannot be realistically applied to ordinary windows of buildings.